Echinorhynchus coregoni is an acanthocephalan parasitic in the digestive tract of coregonid fish. Juveniles of this form have been found in the amphipod, Pontoporeia heyii. In the present work it was found that the eggs of this acanthocephalan will hatch when ingested by Hyalella azteca. The acanthor penetrates the amphipod gut wall and locates just beneath the serosa. In most instances the acanthor ceases to develop, is walled, and is destroyed by cellular reaction of the host. The host reaction consists of giant cells which coalesce to form a syncytium. A small per cent of the infeoted amphipods do not overoome the infection. In these hosts the acanthor enters the acanthella stage and has been followed through eight days of development with no apparent signs of injury. The mechanism of liberation of the acanthor from its embryonic membranes was observed within the amphipod intestine. The acanther of Echinorhynchus coregoni has a prominent rostellum armed with b1adelike hooks. Its body is covered with fixed spines, large at the anterior extremity and progressively deoreasing in size posteriorly. Hyalella azteca appears to be an abnormal intermediate host for Echinorhynchus coregoni, but it serves as a good tool both for the study of the cellular reaotion of the amphipod to an acanthocephalan parasite and as a means of liberating the acanthor from its embryonic membranes tor study.